The detection and analysis of particles, such as charged particles or photons, is important in many fields. High energy physics, for example, generally involves the detection and analysis of charged particles. A variety of medical specialties require detection and analysis of various types of radiation, such as x-rays. Depending on the field and the specific application, different types of information relating to the relevant particles may be required. For example, it may be necessary to detect the spatial position of particles, the arrival time of the particles at one or more spatial positions, the cumulative or individual energy deposited by the particles, and/or the identity of the detected particles.
A wide variety of particle detection systems are available. All particle detection systems, however, ultimately depend on the fact that particles transfer energy to the medium the particles are traversing, generally through the process of ionization or excitation of the atoms in the medium. The particles are sent through a selected detection medium, or detector, thereby transferring energy to the detector. The amount, position, and/or time of generation of this deposited energy is used to obtain desired information on the particles.
The choice of a particle detection system depends on the types of particles to be detected and the type and quality of information to be collected concerning the particles. In fact, one problem of conventional particle detection systems is that the same basic hardware generally can be used only for detection of certain types of particles. A single user may therefore be required to have numerous separate particle detection systems in order to detect different types of particles of interest to that user.
In many applications, it is necessary to detect the spatial position of the incoming particles. Depending on the type of particles and the degree of spatial resolution required, this may be accomplished using a continuous film that is "exposed" or altered in some manner when particles hit it. The degree of exposure at various points on the film is related to the particles that have hit the film. Thus, the particles hitting the film during a selected period of time will create an image on the film that can be examined to obtain desired information. The degree of spatial resolution permitted depends in part on the characteristics of the film.
As an alternative to this type of detector, the detector area may be divided into a two-dimensional array of "pixels," each pixel having a relatively small area, where the total energy deposited in each of the pixels in a given period of time is measured. In pixel-based detection systems, the degree of spatial resolution depends in part on the size of the pixels, with smaller pixel size allowing for higher spatial resolution. Such arrays may be viewed as a large number of individual particle detectors in which each detector consists of a particle interaction region and electronics to read out the energy deposited in the interaction region by the particle. The minimum size of the pixels, and thus the spatial resolution, is limited by the amount of electronic circuitry that is in the pixels.
In many applications, such as conventional medical x-rays, the pattern and cumulative effects of a large number of particles are recorded. In such applications, the precise characteristics and arrival time of individual particles are generally ignored. Each pixel in the resultant image represents a measure of the average number and energy of the particles reaching the detector. In this type of system, the array of detectors is typically read only once per image. Each of the particle detectors accumulates a number of hits during the image recording time. Hence, the output of the array is a function of the average energy deposited per pixel per particle, multiplied by the average number of particles passing through the pixel during the measurement time.
In many applications, it is useful or necessary to take into account the characteristics of the individual particles. The two most useful characteristics are the energy of the particle reaching the detector and the time of arrival of the particle. For example, consider the case of an image made by injecting a patient with a radioactive isotope that emits gamma-rays of a known energy. An image is generated by measuring the gamma rays that leave the patient's body traveling in a specified direction. Each pixel records a measure of the average number of gamma-rays reaching the pixel detector per unit time. The trajectory of some of the gamma-rays will be altered by interactions with the tissue of the patient's body. Ideally, one would like to eliminate these gamma-rays. These gamma-rays are generally distinguishable from gamma-rays that did not have such interactions by their energy; the interacting gamma-rays having lower energies. Hence, by measuring the energies of the gamma-rays that reach the detector and ignoring gamma-rays with energies below a predetermined value, these gamma-rays can be eliminated from the image and a higher resolution image obtained.
Similarly, background noise often affects the resolution of the detection system in terms of both the spatial position and arrival time of particles. Some background noise is caused by excitation of the atoms in the detector that resulted from sources other than the particles of interest. Other background noise is caused by secondary effects from the detected particles that result in unusable data. For example, Compton scattering can result in photons being scattered and detected in a plurality of pixels. In general, noise events can be reduced by eliminating events with energies below a predetermined threshold.
To measure the energy of individual particles, the detector must be read before multiple events are accumulated for any pixel. This is necessary because the data stored in a pixel represents the energy collected by the corresponding pixel detector, and the image processing system generally cannot determine the number of particles generating energy in a given pixel before the data is read out. Hence, in prior art systems, to obtain data on the energy of individual particles, no more than one particle can pass through the pixel detector between read-outs. This constraint requires that the pixel detectors be read out at a high frequency. In prior art systems, this is accomplished by periodically reading out the entire array at a high frequency. If the number of pixels in the image is large, the readout time can be excessive. The readout time determines the maximum flux of particles for which the detector can be used. It is advantageous to use high fluxes to reduce the exposure time, hence, long readout times are to be avoided.
In many applications, recording the time of arrival of the particles at the detector also allows for improved image quality. For example, when generating an image of a medical patient, the natural movements of the body associated with breathing can cause blurring of the image when the time needed to accumulate data for the image takes longer than a small fraction of the breathing cycle. One method for reducing blurring is to correlate the arrival time of particles with specific points in the relevant bodily cycle. For example, by limiting the image to gamma-rays that are detected when the patient has completed inhaling, motion artifacts can be reduced.
In prior art systems, this is accomplished by activating the detector only during the "allowed" time periods. Hence, most of the data available to the detector is not recorded. This leads to inefficient use of the detector and increased image acquisition times. Alternatively, the detector could be read out at a number of different times in each breathing cycle and separately accumulated to provide a number of images that represent the patient at each of a number of points in the breathing cycle. However, this latter strategy imposes a significant computational load on the read-out system, because of the high data rates inherent in reading out the entire array a number of times during each breathing cycle.
As noted above, each pixel detector consists of a region in which the energy is deposited by a particle (i.e., a particle detection region) and a read-out circuit (i.e., a data acquisition region) that is used to generate an electrical signal indicative of the energy deposited by particles since the last read-out. Detectors for different energies and types of particles differ from one another primarily in the particle detection region. Prior art systems, however, are constructed with an single type of particle detection region packaged permanently with an electronic data acquisition system. This increases the cost of systems by reducing the economies of scale that would be available if a single electronic system could be used for a wide variety of detectors.
Accordingly, there is a need for improved electronic data acquisition circuitry for use in a particle detection system that eliminates disadvantages of prior art systems.